The present invention relates to a cooling apparatus for an internal combustion engine that performs diagnosis of an operating state of a thermostat.
In an internal combustion engine, a phenomenon (stuck-open valve) often occurs in which a valve inside the thermostat is in an opened state and yet does not operate. In a state in which the stuck-open valve occurs (at the abnormal state of the thermostat), a coolant is always circulated through a radiator, and therefore, the temperature of the coolant is hard to rise as compared with the normal state of the thermostat.
In a conventional cooling apparatus, anomaly of the thermostat is detected by paying attention to a change of the temperature of the coolant through the following processes (A) and (B).
(A) Based on the parameter having a correlation to the coolant temperature, a reference temperature equivalent to the coolant temperature is calculated when an operating state of the thermostat is normal.
(B) When a diagnosis condition is established, the reference temperature and the actual coolant temperature are compared, thereby to diagnose on the operating state of the thermostat. That is, when a ascending degree of the reference temperature is greater than the ascending degree of the actual coolant temperature, it is determined that an anomaly is developed in the thermostat.
In the cooling apparatus disclosed in Japanese Laid-Open Patent Publication No. 2000-220456, attention is paid to the fact that a difference between the coolant temperature and the external air temperature affects a heat dissipation rate of the coolant. That is, based on that temperature difference, the reference temperature is calculated, so that the anomaly of the thermostat is detected. Further, for the calculation of the temperature difference, the detection value of an intake-air temperature sensor is used as the value equivalent to the external air temperature.
Since the intake-air temperature sensor rises in its temperature by heat reception from the engine, the detection value of the sensor shows values greater than the actual external air temperature.
Hence, in the cooling apparatus disclosed in Japanese Laid-Open Patent Publication No. 2000-220456, in case a ascending degree of the temperature of the intake-air temperature sensor is excessively great, a diagnosis of the operating state is performed based on the reference temperature sharply deviated from a predetermined value, and it is, therefore, not possible to accurately detect the anomaly of the thermostat.
In a case of a cooling apparatus in which the detection value of the intake-air temperature sensor is adopted as the external air temperature, and the reference temperature is estimated based on that detection value, the same problem mentioned above occurs.